A model for the waveform behavior of accreting millisecond pulsars: Nearly aligned magnetic fields and moving emission regions

TL;DR

This paper proposes a model where nearly aligned, moving X-ray emission regions near the spin axes of accreting millisecond pulsars explain their observed waveform properties, including low amplitudes and variability.

Contribution

The nearly aligned moving spot model offers a new explanation for pulsar waveform behaviors, linking magnetic field alignment and accretion dynamics to observed pulsar signals.

Findings

Explains low oscillation amplitudes and sinusoidal waveforms.

Accounts for variability and phase shifts in pulse signals.

Suggests why some pulsars are difficult to detect or intermittent.

Abstract

We investigate further a model of the accreting millisecond X-ray pulsars we proposed earlier. In this model, the X-ray-emitting regions of these pulsars are near their spin axes but move. This is to be expected if the magnetic poles of these stars are close to their spin axes, so that accreting gas is channeled there. As the accretion rate and the structure of the inner disk vary, gas is channeled along different field lines to different locations on the stellar surface, causing the X-ray-emitting areas to move. We show that this "nearly aligned moving spot model" can explain many properties of the accreting millisecond X-ray pulsars, including their generally low oscillation amplitudes and nearly sinusoidal waveforms; the variability of their pulse amplitudes, shapes, and phases; the correlations in this variability; and the similarity of the accretion- and nuclear-powered pulse shapes and phases in some. It may also explain why accretion-powered millisecond pulsars are difficult to detect, why some are intermittent, and why all detected so far are transients. This model can be tested by comparing with observations the waveform changes it predicts, including the changes with accretion rate.